1. Field of the Invention
The present invention relates to a plasma CVD apparatus for uniformly depositing a semiconductor thin film on a substrate, with excellent electric characteristics and to a method for manufacturing a thin film. More particularly, the invention relates to a method and apparatus for continuously fabricating a photovoltaic element comprising a microcrystal thin film on an elongated substrate.
2. Related Background Art
As a method for continuously forming a semiconductor functional deposited film, used for photovoltaic elements or the like, on a substrate, the specification of U.S. Pat. No. 4,400,409 discloses the plasma CVD process employing the roll-to-roll method.
This process is described as being capable of continuously forming an element having a semiconductor junction by using an elongate beltlike member as a substrate, and continuously conveying the substrate in the longitudinal direction thereof while depositing necessary conductivity-type semiconductor layers thereon in a plurality of glow discharge regions.
A schematic diagram of a conventional deposition apparatus is shown in FIG. 2. A beltlike member 2000 is conveyed by magnet roller 2005. When electric power is applied between cathode electrode 2002 and anode electrode 2004, material gas 2003 is decomposed to generate a plasma in a glow discharge space 2006 and to form a film on the beltlike member 2000. In FIG. 2 the arrow 2003 represents the flow of the material gas.
The above apparatus, however, has a problem that it is not easy to maintain uniform discharge states with good reproducibility during a period of several hours for depositing the film over the entire length of the elongated substrate.
Further, a light-incident-side doped layer of amorphous silicon solar cell is desirably microcrystallized due to demand for improvement in optical transparency.
As conventional techniques for forming a microcrystalline p-type semiconductor layer or n-type semiconductor layer, there are a method for mixing phosphine (PH.sub.3), diborane (B.sub.2 H.sub.6), or the like as a dopant in silane (SiH.sub.4) or the like being the material gas and for further diluting it with a large amount of hydrogen (H.sub.2) (in the dilution rate of 10 to 100 or more) and a method for applying high radio-frequency (RF) power, but they were not enough to stably form the microcrystalline film. The reason is that excitation and decomposition of the material gas is promoted only in a certain localized portion in the proximity of the cathode electrode. Also, these methods consume a lot of both raw-material gas and power, thus being disadvantageous from a viewpoint of cost.
Further, there is another conventional technique for positively applying a positive potential (bias) to the cathode electrode using direct-current (DC) power supply or the like. However, since such a system employs the secondary means of DC power supply, it is a system to permit direct current to flow into plasma discharge. Therefore, abnormal discharge such as sparks will occur with increasing DC voltage bias. It was thus very difficult to maintain stable discharge as suppressing the abnormal discharge. Accordingly, it was doubtful whether application of the DC voltage to the plasma discharge was effective. This is because the system is one in which the DC voltage is not separated from the direct current. In other words, it has been desired to have a means for effectively applying only the DC voltage to the plasma discharge.
An object of the present invention is to provide a method and deposition apparatus capable of forming a semiconductor thin film which is spatially uniform and excellent in electric characteristics, with good reproducibility, and at a high deposition rate.